1. Field of the Invention
The present invention relates generally to communication systems which use digital modulation techniques, such as QAM, QPSK and OQPSK, and more specifically to instrumentation for measuring and troubleshooting modulation problems and errors.
2. Description of Related Art
There is presently a shift in modern communication systems from analog modulation methods to digital modulation methods. This is driven by the ever-increasing need to drive more information through narrower channels of the RF spectrum and for computers to communicate between themselves. While the design of each communication system is driven by its own particular needs, there have developed a general set of digital modulation schemes including, M-ary PSK modulation schemes and M-try quadrature amplitude modulation (QAM) schemes. Such methods include, among others, quadrature phase shift keying (QPSK), offset quadrature phase shift keying (OQPSK).
The bandwidth efficiency of these systems may be maximized using a group of theorems developed by Nyquist, including the Nyquist Minimum-Bandwidth Theorem and the Nyquist Intersymbol Interference and Jitter Free Transmission Theorem. The theorems describe a method of spreading the energy of one symbol over the period of time used to transmit several symbols without creating intersymbol interference. These theorems may be implemented using a system of filters which are generally referred to as Nyquist filters. Nyquist filters generate considerable distortion except in the regions in the immediate neighborhood of the "sampling instants" or "null points" determined by a symbol clock. If the symbol clock is known, including both its rate and phase, then the symbol clock may be used to recover the binary signal and to drive a display to generate eye diagrams and constellation diagrams. The size of the eye in the eye diagram and the stability of the constellation diagram are useful measures of the communication system operating margins. Unstable eye patterns and constellation diagrams may be used to diagnose modulation or other system problems, such as unstable carriers.
Digital demodulators for M-ary PSK modulated signals must use coherent detection. Coherent detectors require an exact replica of the carrier, in both frequency and phase. Any errors in this carrier replica, either in frequency or phase, can cause significant errors in the later stages of the receiver. Coherent detection of digitally modulated signals is subject to a well known carrier phase ambiguity. This may be most easily demonstrated by considering the constellation for a particular modulation scheme. If the constellation is rotated by phase errors between the actual carrier and the replica carrier, for certain angles of rotation, there is no way that the demodulator in the receiver can distinguish the rotated constellation from a valid constellation without some knowledge of the data. These ambiguities may be overcome by additional information from the data itself, for example, a short preamble, or through differential coding. Certain modulation schemes have an inherent symbol phase ambiguity, for example, OQPSK. For OQPSK, there are two clock transitions, an I transition and a Q transition, causing symbol phase uncertainty. Symbol phase ambiguities are additive with the carrier phase ambiguities complicating demodulation. When coherent demodulators are used to demodulate these signals, additional information from the data itself or differential coding must be used to remove the ambiguity.
Since in most systems the receiver timing is independent of transmitter timing, the carrier frequency and phase must be derived from the transmitted signal. Generally, the replica carrier is generated using a narrow band phase-locked loop. Two general approaches exist to provide a phase reference to "lock" this loop, conventional approaches and data directed approaches. Conventional approaches, for example, frequency doubling, frequency quadrupling and Costas loops, multiply the transmitted signal in order to generate a coherent phase reference. Data directed demodulators derive symbol timing information from the signal and use that information to generate a coherent phase reference. Conventional approaches require a long data transmission to generate the coherent phase reference. In modern digital communication systems, short bursts are increasingly being used. These burst are often too short for conventional approaches to "lock". The data directed approaches can obtain the required phase information more quickly and therefore will "lock" more quickly. Therefore, data directed approaches are also better suited for test equipment.
Existing data directed approaches have relied on the existence of a spectral component or "spectral lines" in the modulated signal or on a unique segment or "preamble" in the modulated signal to acquire the needed symbol timing information. However, modulation schemes and some system situations are incompatible with these approaches. Staggered M-ary PSK modulated signals do not have a useable spectral line component, because it is very weak. Many modulated signals do not have long preambles. Finally, any distortion and noise in the transmission system can sometimes cause problems detecting the spectral information. Therefore, there exists a need for a more robust technique for acquiring the required symbol timing information for data directed demodulators.